Pneumatic tires sometimes have treads of a running surface comprised of a carbon black-rich rubber composition or of a silica-rich rubber composition.
Pneumatic tires having treads of a running surface comprised of a silica-rich rubber composition, while sometimes desirable to impart various physical properties to the tire tread such as, for example reduced rolling resistance and suitable traction, may be disadvantageous because of the relatively high ratio of silica to carbon black in the reinforcing filler content. Such high silica/carbon black ratio represents a significant increase in cost of the silica-rich tread in terms of increased material cost (the silica) and increased cost of processing the silica-rich rubber composition. Further, such silica-rich tread rubber, with its minimal carbon black content, is of a relatively low electrical conductivity and is therefore electrically resistive to conducting static electricity from the tire tread to the ground.
Sometimes a path of increased electrical conductivity for a silica-rich tread may be provided, for example, by positioning a strip of a carbon black rich rubber composition either as a thin cover strip over a portion of the running surface of the tread or as a thin, non load bearing, strip extending through the body of the tread to its running surface. Methods of such type add both cost and complexity to the tire itself and to the manufacturing procedure for the tire.
For this invention, in order to both reduce the material and fabrication cost of a silica-rich tread and to provide a path of increased electrical conductivity from the tire through its tread to the ground, it is envisioned that the tread running surface be divided into three distinct load bearing zones which include at least one silica-rich load bearing zone and at least one carbon black-rich load bearing zone of rubber compositions.
Historically, a tire tread has heretofore been suggested having a running surface composed of three longitudinal portions namely, two black colored lateral portions and a non-black colored central portion located between the two black portions, wherein the lateral black colored portions have wear resistant properties virtually identical to the central colored portion (EP 0 993 381 A3, FR 2765525 and WO 99/01299 patent publications).
However, for this invention, it is desired to provide a tire tread of three distinct, and significantly wide, circumferential load-bearing zones, each of which contain carbon black reinforcement and are thereby black in color.
Historically, U.S. Pat. No. 5,225,011 relates to a tire having a tread composed of a center rubber composition and side rubbers (FIG. 1) positioned directly onto a tire carcass belt without a tread base transition layer. The center rubber is required to be limited to either natural rubber or a natural rubber/styrene-butadiene rubber blend. The center rubber contains a carbon black of large iodine absorption number of at least 100 mg/g, silica and silane coupling agent and the side rubbers are required to be of a different rubber composition.
Historically, European patent publication number EP 864,446 A1 relates to a tire having a tread (FIG. 2) with a central portion (B) and side portions (A) positioned directly onto a tire carcass belt without a tread base transition layer. The side portions are carbon black rich and the central portion is silica rich, wherein the silica content of the central portion (B) is at least 20 percent higher than in the side portions (A).
For the zoned tread of this invention, by requiring the tread cap zones to be load-bearing, it is meant that each of the three distinct running surface tread cap zones extend from the outer surface of the tread to the underlying carbon black-rich tread base rubber composition so that all of the load on the tire is communicated by each of the three tread cap layer zones directly to the tread base layer instead of directly to remainder of the tire carcass itself.
By requiring that each of the running surface tread zones be significantly wide, and therefore each comprising a significant portion of the tread running surface, it is intended that each respective zone more effectively transmits the load from the outer surface of the running surface of the tire to the tread base layer. For such purpose of this invention, the silica-rich zone, or zones, comprise from about 55 to about 80 percent of the width of the of the tread cap and, correspondingly, the carbon black-rich zone, or zones, comprise from about 20 to about 45 percent of the width of the tread cap.
It is an additional aspect of the zoned tread configuration of this invention that the tread running surface of the load bearing carbon black-rich zone(s) provide a path of increased electrical conductivity from the tire to the ground.
Such structural, zoned, load-bearing, configuration of a tire tread running surface is considered herein to be a substantial improvement of providing a tread with a path of increased electrical conductivity to the tread running surface (the tread surface intended to be ground-contacting) from the carbon black rich tread base layer without resorting to, for example, providing a narrow strip of electrically conductive rubber through an electrically insulative tread running surface (e.g silica-rich tread cap surface) or providing a thin layer of electrically conductive rubber composition over a portion of the outer surface of the tread cap running surface or providing a thin layer of electrically conductive rubber composition around a periphery of a silica rich tread. Such layers are typically required to be thin in order to not unnecessarily interfere with the performance of the basic tire tread running surface. For example, see U.S. Pat. Nos. 5,942,069, 6,044,882, 5,872,178, 6,415,833, 6,140,407, 5,518,055 and 5,882,456. However, such thin layers are not considered herein to be significantly load-bearing in the manner required by the substance of the respective, relatively wide, distinct tread cap zones of this invention for transmitting significant loads from the running surface of the tread to the underlying tread base layer.
It is considered herein that utilization of such thin layers, or strips, of electrically conductive rubber compositions unnecessarily add to the cost and complexity of the tire tread and its process of fabrication.
Therefore, in one aspect of the invention, it preferred that the tread cap layer is exclusive of a thin, electrically conductive, carbon black rich rubber strip positioned through the tread cap layer, such as from a carbon black rich tread base through a more electrically resistive tread cap silica rich rubber composition based zone; over the tread cap rubber layer and therefore between the tread cap rubber layer and the ground; or around the tread cap rubber layer extending from a portion of its running surface to a carbon black rich component of the tire such as for example a tread base layer or a tire sidewall.
In the description of this invention, the terms “rubber” and “elastomer” where used herein, are used interchangeably, unless otherwise provided. The terms “rubber composition”, “compounded rubber” and “rubber compound”, if used herein, are used interchangeably to refer to “rubber which has been blended or mixed with various ingredients and materials” and such terms are well known to those having skill in the rubber mixing or rubber compounding art.
In the description of this invention, the term “phr” refers to parts of a respective material per 100 parts by weight of rubber, or elastomer. The terms “rubber” and “elastomer” may be used interchangeably unless otherwise provided. The terms “cure” and “vulcanize” may be used interchangeably unless otherwise provided.
In the description of this invention, the elastomer Tg may be determined by differential scanning calorimeter (DSC), an industry standard test, at a heating rate of 10° C. per minute.